Which size for most efficient Boiler?

[Agile]

I have to say that I am not at all impressed by him when he chooses a 37 kW boiler when a 24 kW will more than suffice.

Do a few searches on "oversized boiler" and you will read just how problematic that can be, particularly on Vaillants.

[ I wonder what is "Vaillant" about the engineer? ]

Tony

 

[Darems]

"oversized boiler" and you will read just how problematic that can be.

I did and that's exactly why I went on this journey to find the right sized boiler myself (aside from wanting to be fuel & cost efficient):
undersized will over-stress the system and over-sized will damage it through excessive short-cycling

The engineer is Vaillant registered which means we get the full 10 years warranty ...

 

[Razor900]

Bearing in mind that we haven't seen it and he has I still think the boiler is very oversized which brings it's own set of problems.

If you want a second quotation and you're in North London I would totally recommend you contact Dan Robinson who posted above through the forum.

I'm sure he would give you a like for like Vaillant quote if you insisted but he may even have better alternatives to discuss with you!

 

[Steelmasons]

One particular model of Vaillant caused issues when oversizing (4xx series) , this was supposedly ironed out due to an upgrade in the software.
I've oversized a good few boilers due to forthcoming extension etc......range rating a Vaillant 637 down to 24kw is no problem whatsover although heat exchanger resistance will have to be taken into account.

 

[Steelmasons]

"oversized boiler" and you will read just how problematic that can be.

I did and that's exactly why I went on this journey to find the right sized boiler myself (aside from wanting to be fuel & cost efficient):
undersized will over-stress the system and over-sized will damage it through excessive short-cycling

The engineer is Vaillant registered which means we get the full 10 years warranty ...

 

[Razor900]

Oh it'll work ok Steel but if the boiler is oversized the client will end up paying more for a larger gas supply and the boiler won't modulate low enough to satisfy the demand without excessive cycling when the system is up to temp.

I've lost count of the number of our 40kW boilers I've seen installed in properties with 8 rads and 22mm primary pipework - it pizzes me right off

 

[Dan Robinson]

Yes, but as it's on a different circuit from the CH I was wondering how the boiler knows.

By appropriate wiring and controls.

I don't trust any of these online calculators. They're all aimed at flogging you the biggest and most expensive model they can. Properly calculated and designed systems use much smaller boilers than people realise.

We're doing a 236 square metre house at the moment. All UFH. Heatloss calculated to 15kW.....4 storey terrace town house.

 

[Dan Robinson]

Oh it'll work ok Steel but if the boiler is oversized the client will end up paying more for a larger gas supply and the boiler won't modulate low enough to satisfy the demand without excessive cycling when the system is up to temp.

I've lost count of the number of our 40kW boilers I've seen installed in properties with 8 rads and 22mm primary pipework - it pizzes me right off

The one I referenced to above wanted an SB40 fitted. I tried to get him to have an Eco RF24 but he refused point blank - he agreed to an RF30.... He thought (naturally enough I suppose) that the numbers referred to the actual kW's


Who am I to argue?

 

[Steelmasons]

Oh it'll work ok Steel but if the boiler is oversized the client will end up paying more for a larger gas supply and the boiler won't modulate low enough to satisfy the demand without excessive cycling when the system is up to temp.

I've lost count of the number of our 40kW boilers I've seen installed in properties with 8 rads and 22mm primary pipework - it pizzes me right off

Hey Razor , Interslash should take a leaf out of Vokeras book with regard to the 10:1 turndown ratio..

Vaillant 637 should modulate down to around 7kw..
What's the turndown ratio on your beloved Interslash?
Changing DSN on the MK1 Ecotec was a god send..

 

[Dan Robinson]

Every beauty has her blemish.

 

[Steelmasons]

A match made in heaven......Razor working for Interslash with Dan fitting em......

 

[Steelmasons]

Every beauty has her blemish.

....and another fooked keyboard..

 

[Razor900]

I dunno Steel, did you mean the 8 year old model or the new one that's on it's way??

Or the one due in Sept that has been rated at 122% efficiency???

 

[Steelmasons]

Can't compete with the Interslash ......efficiency now rated @ 199%???..fook me it's gotta be a steal...
How many moving parts have they got now??...2??...

 

[Darems]

OK guys - you've lost me now, but good banter
If I raise external to -3C and only allow 10% for heat up I end up with a heat loss of 13KW (I've made a few more adjustments too (like no external walls in the loft)

What result do you get if you use the Baxi Whole House Boiler Size Calculator? (It's not biased as it uses the BRE method.)

I wasn't able to use the BAXI calculator as it was asking for extra sheets, and it gets a bit confusing measuring side walls include single story extensions...
I really like the Myson calc because it breaks down by room, so I can decide on which Rads are required

I've started tweaking the results to make the workbook dynamic - attached

Thanks